1. Field of the Invention
This invention relates to the treatment of waste water containing biological impurities (e.g. sewage) to break down the impurities by reaction with micro-organisms and thereby render the waste water less noxious and more easily disposable (e.g. in natural water courses).
In particular, the invention relates to a biomass-containing waste water treatment plant which comprises at least one first stage in which the biomass is subjected to digestion by microorganisms and in which an initial separation between solid and liquid constituents of the waste water is effected and a final settlement zone in which residual solids can settle out from the liquid phase of the treated waste water.
2. Description of the Prior Art
Typically there are two preliminary stages, the first being a primary settlement zone that receives the waste water from the source thereof and feeds the water (after removal of the heavier solids therefrom) to a bio zone where the aforesaid biomass digestion occurs, usually on the surface of slowly rotating rotors which become coated with the biomass and periodically lift the biomass out of the carrier water to transport it into the oxygenrich atmospheric environment. The liquid phase, leaving the bio zone, flows into the final settlement zone.
Normally the waste water flows by gravity from the at least one first stage to the final settlement zone so that the free liquid level in the latter zone is slightly below the free liquid level in the bio zone. Some sludge settlement occurs in the final settlement zone and this has to be removed from time to time. Desludging of the primary settlement zone is also effected periodically and there are operating advantages to be gained from transferring the sludge from the final settlement zone to the at least one first stage, and, where such is provided, into the primary settlement zone.
With the normal operating hydraulic levels, transferring sludge from the final settlement zone upstream thereof, involves a pumping operation and it is known to provide a sludge pump to overcome the natural head differential that exists. Such a sludge pump (which can be mechanical or air lift) needs to be provided, maintained and powered, thus adding to the installation costs, running costs and servicing costs of the waste water treatment plant.
This invention relates to an improved method of operating a waste water treatment plant and to an improved water treatment plant in which the need for such a sludge pump is avoided.